Code that cannot be copyrighted is not proprietary code: you can't leverage copyright law(s) to forbid its copies, but there are also no obligations whatsoever to release it.
Code that cannot be copyrighted is also not FLOSS code: the way "copyleft" code has always worked was via leverage of copyright law(s) by counterpoint: your copyright laws forbid copies, we use your rules to enforce certain rights.
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